Segment Information - Unconsolidated Real Estate Ventures of Company's Reportable Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Segment Reporting Information [Line Items]
Investment in joint venture	$ 422,333		$ 422,333		$ 389,811
Equity in loss of real estate ventures	(7,240)	$ (2,203)	(14,164)	$ (4,094)
Philadelphia CBD
Segment Reporting Information [Line Items]
Investment in joint venture	310,237		310,237		268,562
Equity in loss of real estate ventures	(4,661)	83	(8,940)	161
Metropolitan Washington, D.C.
Segment Reporting Information [Line Items]
Investment in joint venture	98,839		98,839		99,769
Equity in loss of real estate ventures	(778)	(647)	(1,195)	(1,096)
Mid-Atlantic Office Joint Venture
Segment Reporting Information [Line Items]
Investment in joint venture	32,388		32,388		32,996
Equity in loss of real estate ventures	117	0	324	0
MAP Venture
Segment Reporting Information [Line Items]
Investment in joint venture	(19,131)		(19,131)		$ (11,516)
Equity in loss of real estate ventures	$ (1,918)	$ (1,639)	$ (4,353)	$ (3,159)